1. Field of the Invention
The present invention relates to a member coated with a hard coating to attain excellent wear resistance. More specifically, the present invention relates to a member coated with a hard coating intended for use in wear-resistant members such as cutting tools employed in milling, cutting, drilling operations etc., molds, bearings, dies, rollers for which a high level of hardness is demanded, and heat-corrosion resistant members such as screw of extruders, cylinders and the like.
1. Description of the Background
The formation of a hard coating such as TiN and TiCN on the surface of a tool in order to improve the wear resistance of the tool is a technique commonly used in the field of cutting tools for which a high level of wear resistance is demanded such as high speed steel tools, cemented carbide tools etc.
There is also a hard coating made of a carbonnitride composite of Ti and any metal in the group 4A, 5A, and 6A metals other than Ti. The hard coating of this composition is suggested, for example, in Japanese Unexamined Patent Publications Nos. 60-248879, 4-221057, and 7-173608, as a coating having superior properties than the TiN and TiCN hard coatings. Specifically, the carbonnitride composite coating has a composition of (Ti, M)(N, C) (where M is Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, or W). The carbonnitride composite coating suggested in these prior arts shows excellent wear resistance; however, its adhesion is poorer than that of TiN and TiCN coatings. Due to the poor adhesion, there is a problem that the coating peels off from the base material during use.
In an attempt to give enhanced adhesion to the hard coating, Japanese Unexamined Patent Publication No. 10-158861 suggests to form an intermediate layer of TiN between the hard coating and the base material. The TiN intermediate layer, however, is not effective in increasing the adhesion of the hard coating to a sufficient level. Rather, the TiN intermediate layer causes another problem that it impairs the wear resistance. This is because the hardness of the TiN intermediate layer is low. The TiN intermediate layer with low hardness defeats the effect of the (Ti, M)(N, C) coating with high hardness. This disadvantage is especially pronounced in the case where this technique is applied to a cutting tool as a base material. In the case of cutting a material with low hardness such as S50C, a cutting tool coated with a (Ti, M)(N, C) hard coating exhibits substantially the same wear resistance as that coated with a (Al, Ti)N hard coating. The (Al, Ti)N hard coating recently receives attention for its excellent wear resistance. However, in the case of cutting a material with high hardness such as JIS (Japanese Industrial Standard) SKD 61 hardened steel, the cutting tool coated with the (Ti, M)(N, C) coating exhibits poor wear resistance as compared with that coated with the (Al, Ti)N coating.
The (Al, Ti)N coating also has a problem. As has described above, the (Al, Ti)N coating recently receives an attention as a coating for cutting tools intended for cutting steel materials of high hardness and for use in high-speed cutting operations. It is true that cutting tool coated with (Al, Ti)N coating exhibits higher wear resistance than that coated with (Ti, M)(N, C) when it is used for cutting steel materials of high hardness such as JIS SKD61 hardened steel. However, when the cutting tool coated with (Al, Ti)N coating is used for cutting steel materials of low hardness such as S50C carbon steel, there are some cases where it exhibits poor wear resistance as compared with that coated with (Ti, M)(N, C) coating depending on cutting conditions.
Besides a tertiary composite material such as (Al, Ti)N, it is also suggested to use a quaternary composite material in order to form a coating with excellent wear resistance. For example, Japanese Unexamined Patent Publications Nos. 3-120354, 10-18024, and 10-237628, and Japanese Examined Patent Publication No. 5-88309 disclose an (Al, Ti, V)N hard coating. The cutting tool coated with the (Al, Ti, V)N hard coating exhibits high wear resistance in the case of cutting steel materials with low hardness such as JIS S50C carbon steel. However, the (Al, Ti, V)N coating has a problem that it has poor adhesion to a base material, and therefore, easily peels off. In order to improve its adhesion, a TiN intermediate layer is formed between the coating and the base material, or the composition of Al and V of the coating is gradually changed along the its thickness direction. However, these techniques do not sufficiently improve the adhesion of the coating. Rather, the cutting tool coated with the (Al, Ti, V)N coating exhibits poor wear resistance as compared with those coated with the (Al, Ti)N coating in the case of cutting work materials with high hardness such as JIS SKD61 hardened steel.